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LM5035C

SNVS631D – JANUARY 2010 – REVISED OCTOBER 2016

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8.3 Feature Description

8.3.1 High-Voltage Start-Up Regulator

The LM5035C contains an internal high-voltage start-up regulator that allows the input pin (VIN) to be connected

directly to a nominal 48-VDC input voltage. The regulator input can withstand transients up to 105 V. The

regulator output at VCC (7.6 V) is internally current-limited to a minimum of 58 mA. When the UVLO pin potential

is greater than 0.4 V, the VCC regulator is enabled to charge an external capacitor connected to the VCC pin.

The VCC regulator provides power to the voltage reference (REF) and the output driver (LO). When the voltage

on the VCC pin exceeds the UVLO threshold of 7.6 V, the internal voltage reference (REF) reaches its regulation

setpoint of 5 V and the UVLO voltage is greater than 1.25 V, the controller outputs are enabled. The value of the

VCC capacitor depends on the total system design, and its start-up characteristics. The recommended range of

values for the VCC capacitor is 0.1 µF to 100 µF.

The VCC undervoltage comparator threshold is lowered to 6.2 V (typical) after VCC reaches the regulation

setpoint. If VCC falls below this value, the outputs are disabled, and the soft-start capacitor is discharged. If VCC

increases above 7.6 V, the outputs will be enabled and a soft-start sequence will commence.

The internal power dissipation of the LM5035C can be reduced by powering VCC from an external supply. In

typical applications, an auxiliary transformer winding is connected through a diode to the VCC pin. This winding

must raise the VCC voltage above 8.3 V to shut off the internal start-up regulator. Powering VCC from an

auxiliary winding improves efficiency while reducing the controller’s power dissipation. The undervoltage

comparator circuit will still function in this mode, requiring that VCC never falls below 6.2 V during the start-up

sequence.

During a fault mode, when the converter auxiliary winding is inactive, external current draw on the VCC line

should be limited such that the power dissipated in the start-up regulator does not exceed the maximum power

dissipation of the IC package.

An external DC bias voltage can be used instead of the internal regulator by connecting the external bias voltage

to both the VCC and the VIN pins. The external bias must be greater than 8.3 V to exceed the VCC UVLO

threshold and less than the VCC maximum operating voltage rating (15 V).

8.3.2 Line Undervoltage Detector

The LM5035C contains a dual level undervoltage lockout (UVLO) circuit. When the UVLO pin voltage is below

0.4 V, the controller is in a low current shutdown mode. When the UVLO pin voltage is greater than 0.4 V but

less than 1.25 V, the controller is in standby mode. In standby mode the VCC and REF bias regulators are active

while the controller outputs are disabled. When the VCC and REF outputs exceed the VCC and REF

undervoltage thresholds and the UVLO pin voltage is greater than 1.25 V, the outputs are enabled and normal

operation begins. An external setpoint voltage divider from VIN to GND can be used to set the minimum

operating voltage of the converter. The divider must be designed such that the voltage at the UVLO pin will be

greater than 1.25 V when VIN enters the desired operating range. UVLO hysteresis is accomplished with an

internal 23-µA current sink that is switched ON or OFF into the impedance of the setpoint divider. When the

UVLO threshold is exceeded, the current sink is deactivated to quickly raise the voltage at the UVLO pin. When

the UVLO pin voltage falls below the 1.25-V threshold, the current sink is enabled causing the voltage at the

UVLO pin to quickly fall. The hysteresis of the 0.4-V shutdown comparator is internally fixed at 100 mV.

The UVLO pin can also be used to implement various remote enable and disable functions. See Soft Start for

more details.

8.3.3 Line Overvoltage, Load Overvoltage, and Remote Thermal Protection

The LM5035C provides a multipurpose OVP pin that supports several fault protection functions. When the OVP

pin voltage exceeds 1.25 V, the controller is held in standby mode, which immediately halts the PWM pulses at

the HO and LO pins. In standby mode, the VCC and REF bias regulators are active while the controller outputs

are disabled. When the OVP pin voltage falls below the 1.25-V OVP threshold, the outputs are enabled, and

normal soft-start sequence begins. Hysteresis is accomplished with an internal 23-µA current source that is

switched ON or OFF into the impedance of the OVP pin setpoint divider. When the OVP threshold is exceeded,

the current source is enabled to quickly raise the voltage at the OVP pin. When the OVP pin voltage falls below

the 1.25-V threshold, the current source is disabled causing the voltage at the OVP pin to quickly fall.

Several examples of the use of this pin are provided in Application Information.